1. Field of the Invention
This invention relates to cellular telephone systems and, more particularly, to computer implemented processes for predicting interference at mobile units between frequency channels provided by different base stations (cells) in a pattern of base stations.
2. History of the Prior Art
Presently available commercial mobile communication systems typically include a plurality of fixed base stations arranged in patterns in which each base station transmits and receives a number of frequencies. In the most prevalent American Mobile Phone System (AMPS) system, a frequency band of 25 MHz providing approximately four hundred different adjoining FM frequency channels has been allotted by the federal government to each cellular operator. A mobile unit within range of the base station communicates with the external world through the base station using these channels. In a typical system, each channel uses a FM frequency band width of 30 KHz. for downlink transmission from a base station (cell) to a mobile unit and another 30 KHz. for uplink transmission from a mobile unit to a cell. Typically, the frequencies assigned to the downlink transmissions for a cellular system immediately adjoin one another and are widely separated from the frequencies assigned to the uplink transmissions for a cellular system which also immediately adjoin one another. In this specification, even though widely separated, the pair of frequencies used for both downlink and uplink transmission are generally intended when reference is made to a channel unless the context indicates otherwise.
The channels used by a base station are separated from one another in frequency sufficiently that signals on any channel do not interfere with signals on another channel from that base station. To accomplish this, an operator typically allots a base station a set of channels with frequencies which are each separated from the next by some large number (e.g., twenty-one) channels carrying intermediate frequencies. Thus, in a system with twenty-one channel separation, one base station might have channels 1, 22, 43, 64, 85, and so on up to a total of from five to one hundred individual channels. So long as a mobile unit is within the area in which the signal from a base station is strong enough and is communicating with only that base station, there is no interference with the communications.
However, in order to allow mobile units to transmit and receive telephone communications over a wide area, each cell is normally positioned with its area of coverage adjacent and overlapping the areas of coverage of a number of other cells. When a mobile unit moves from an area covered by one base station to that covered by another, the communication is transferred from one base station to the other in an area where the coverage from different cells overlaps. Because of this overlapping coverage, the channels allotted to the cells are carefully selected so that adjoining cells do not transmit or receive on the same frequencies. The channels used by adjoining base stations are also supposedly separated from one another in frequency sufficiently that signals from any base station do not interfere with signals from another nearby base station. This is typically accomplished by assigning channels to some central cell which are widely separated in frequency in the manner described above, and then assigning channels to the cells surrounding that central cell using a pattern which increases each channel number by one for each sequential cell surrounding the central cell. Thus, if cells are arranged in a honeycomb pattern in which six cells surround a central cell using the above-described channels, a first cell adjacent to the central cell may have channels 2, 23, 44, 65, 86, and so on while a second cell adjoining the central cell may have channels 3, 24, 45, 66, 87, and so on. The pattern of channel assignments continues similarly in the other cells adjoining the central cell. It will be noted that separating each channel assigned to any cell from the next channel assigned to that cell in this manner allows a total of twenty-one cells having entirely different frequencies to be positioned in a system before any frequency must be repeated. The pattern is often called a frequency reuse pattern and may vary in many ways from the specific pattern described.
In some systems, especially those with cells in urban areas carrying heavy traffic, each cell is further divided into three sectors each of which may include channels having the above-described frequency allotment of channels. The antennas of each sector are typically arranged to provide 120 degree coverage. With slightly over four hundred channels available, this allows a repeating pattern of groups of cells in the beehive hexagonal arrangement described above with seven cells each having three sectors. When cells are discussed herein, sectors are normally meant as well unless the context indicates otherwise.
In theory, this form of cell arrangement and channel assignments allows the frequency reuse pattern to be repeated at distances sufficiently separated to negate interference between mobile units on the same and adjacent channels.
Unfortunately, interference does occur for a number reasons. Antenna patterns, power levels, scattering, and wave diffraction differ from cell to cell. Buildings, various other structures, hills, mountains, foliage, and other physical objects cause signal strength to vary over the region covered by a cell. Consequently, the boundaries at which the signal strength of a channel falls below a level sufficient to support communications with a mobile unit vary widely from cell to cell. For this reason, cells adjacent one another do not, in fact, typically form the precise geometric boundaries suggested above. Since cell boundaries must overlap to provide complete coverage of an area and the boundaries of cells are imprecisely defined, overlapping coverage often occurs between channels which might interfere with one another.
For example, the transferring of a communication with a mobile unit from one cell to another is accomplished by a process referred to as a "hand-off." Each cell site has a scanning receiver which measures signal levels sent from a mobile unit. When the received signal strength falls below a threshold level, a cell connected to a mobile unit sends a message to a switching center which controls the area covered by the cell. The switching center includes software which maintains data tables which list the switching thresholds, channels assigned to cells, and the positions of the cells in the system. The switching center sends a message to the neighboring cells which overlap the cell connected to the mobile unit asking the neighboring cells to use their scanning receivers to measure the signal level being sent by the mobile unit on the particular channel. The neighboring cells each measure the signal level and also measure some identification signal (e.g., a supervisory audio tone (SAT) which is sent in an AMPS system on the channel at a frequency above the level of the audio of the mobile channel) to identify the signal from a particular mobile unit. All of the cells report back the signal levels received to the switching center. The switching center software determines whether any neighboring cell is receiving a stronger signal from the mobile unit than the cell presently carrying the mobile transmission. If a cell is receiving a significantly stronger signal (set at a value determined by the switching software), the switching software signals the serving cell to notify the mobile unit to switch to a new channel. The switching software sends another message to the neighboring cell receiving the stronger signal telling it to commit a new channel to the mobile. The first connection is disabled, and the mobile unit tunes to the new channel.
To carry out this hand-off process, the signal levels provided at the boundaries of adjacent cells must each be strong enough to support transmission; or the transmission with a mobile unit will be interrupted. Since the pattern around each cell defined by the signal strength just insufficient to support transmission with that cell is not a neat geometric shape, overlapping occurs in widely varying patterns.
The odd shaped boundaries of the individual cells and the necessity that the cells overlap means that signals on the same channel will often interfere with one another even though they are generated by cells which are at distances theoretically sufficient to eliminate interference. This is especially true when a sectored cell pattern is used because the cells are much closer to one another than in a simple cell pattern. A first signal on a channel from a remote cell interferes with a second stronger signal carrying a mobile transmission on the same channel within the coverage area of a cell when the drop in strength of the first signal from the second signal is less than some threshold level (typically measured in decibels). Moreover, signals on adjacent channels are carried by abutting cells in accordance with the above-described frequency reuse pattern. Typically, frequency filtering is insufficient to eliminate adjacent frequencies entirely; consequently, there may be adjacent channel interference. A signal from another cell on a channel at a frequency adjacent the frequency of a channel carrying a mobile transmission interferes when the strength of the adjacent channel signal is greater than a second threshold level compared to the second signal. when the drop in strength of the interfering signal from the serving signal is less than some second usually higher threshold level; normally an adjacent channel may be closer in strength to a serving channel because frequency filtering eliminates some substantial portion of the adjacent frequency signal.
In order to overcome interference when designing or improving the coverage of a mobile cellular system, it has been typical for a cellular system operator to use predictive software to determine what signal strength is to be expected from each of a particular set of cells. This software utilizes data describing the physical characteristics of the terrain surrounding each cellular site and the physical characteristics of the cellular station to plot expected signal strengths around a cellular site. These signal strength predictions are then overlaid on a graphical plot to determine where antennas should be placed to provide optimum coverage with appropriate overlapping areas for hand-offs. Deciding upon optimum coverage may require that the operator move the points at which individual cells are positioned and use the predictive software to replot expected signal strengths around a cellular site to eliminate expected interference. Once the antenna sites have been determined, the operator assigns channel groups to the cells in accordance with the technique described above.
It is possible that expected interference cannot be eliminated by overlaying signal strength predictive plots; and the operator may place test antennas in position using the results of these predictive mappings and test to determine if actual interference exists. Alternatively, the predictive plots may predict that no interference is likely to occur, and the operator may place the antennas to be used in the system. The operator may later find that interference, in fact, occurs at some positions and test to determine how to eliminate the interference. The determination of whether interference actually exists and where it exists is made by drive tests which measure signal strength of channels at the positions either where a graphical plot shows that interference might occur within the cellular system area or where interference has been reported. Since the predictive software provides only an educated guess at the actual signal strengths produced by the channels at a cell, the points at which interference is predicted are often inaccurately placed. In fact, the predictive software provides an estimation of signal strength with a typical error having a standard deviation of 8 dB. This error is very large compared to the required design tolerances which may range from 3 dB to 17 dB.
During the drive tests to determine actual interference, a single channel at each cell or sector of a cell involved in the interference testing is enabled. A mobile unit with a scanning receiver drives over the roads and highways of the system. The scanning receiver scans and measures the strength of the frequency transmitted by each cell at the points of possible interference as the mobile unit moves. This provides strength measurements of signals generated by each cell at each test point. These strength signals are then plotted on the graphical plot against the cells from which they are believed to emanate. Thus, the expected interference points on channels from different cells which actually interfere with one another may be viewed graphically to determine whether sufficient interference exists to change the channel group assigned to the particular area. As may be seen, this requires a substantial amount of time and limits the interference determination to those points at which a system operator expects to find interference, an expectation the accuracy of which is very suspect.
If the number of points of interference for a cell are sufficiently great, the patterns of channels are changed. That is, the frequency group assigned to a cell (or cells) is typically changed in its entirety to another frequency group in which channels which would interfere with channels carried by other cells do not occur. Sometimes, interference may be eliminated by changing the cell characteristics (such as antenna tilt or power used in particular cells) without changing the channels used. Once channels have been assigned to cells which provide acceptable coverage and expected interference has been eliminated, the system is fixed and operated.
This method of placing cells, assigning frequencies, and eliminating interference is quite slow and labor intensive. Moreover, it does not provide a complete understanding of interference which actually exists in a system since typically only those positions at which prediction software leads the operator to believe that interference may exist are tested to determine whether interference actually exists. In addition, whenever the physical pattern of individual cells in a system changes, the computation of predicted cell coverage, graphical plotting of predicted cell coverage, drive testing to check actual interference, actual interference plotting, and assessment of actual interference takes place over again. This is a very labor intensive process which greatly increases the costs of creating and maintaining mobile systems without guaranteeing that interference will be eliminated. Because of the emerging nature of the market for cellular telephones, those system changes are taking place constantly and at an accelerating rate.
It is desirable to provide a process by which interference between cellular telephone system channels operating at the same frequency and adjacent frequencies may be accurately predicted over each entire cell of an entire system before physical changes are made to the system.
It is desirable to remove most of the labor intensive operations in the prediction of channel interference from the design of cellular telephone operating systems.